The present invention relates to a cable testing technique for locating a discontinuity point of a cable and more particularly to such a technique for enhancing the measurement accuracy of the cable testing technique on the basis of the swept frequency domain reflectometry.
A spectrum analyzer is a typical frequency domain measurement instrument which provides frequency spectrum information in the form of a graph having frequency and amplitude axes. There is a conventional measurement technique using such a frequency domain instrument to locate a fault or discontinuity point of a coaxial cable. This method requires not only a spectrum analyzer but also a tracking generator which generates a frequency matching the swept frequency of the local oscillator of the spectrum analyzer. The output frequency of the tracking generator can be controlled or offset by using a tracking adjustment knob relative to the swept frequency of the spectrum analyzer's local oscillator. The input terminal of the spectrum analyzer and the output terminal of the tracking generator are coupled to one end of a cable. At first an operator adjusts the tracking knob of the tracking generator to set it to a "zero offset" state at which the frequency difference between the local oscillator of the spectrum analyzer and the tracking generator output is equal to the center frequency of the bandwidth of the filter of the analyzer, so that a first maximum rise of the base line is displayed on the screen of the spectrum analyzer. As the operator increases the output frequency of the tracking generator by adjusting the offset frequency relative to the local oscillator frequency, the analyzer's display response begins to fall because the frequency difference between the local oscillator and the tracking generator's output is out of the bandwidth of the filter of the spectrum analyzer. The further increase of the tracking generator's output frequency causes the analyzer to produce a base line rise again and reach a second maximum response. This second maximum response is produced by the delayed signal reflected from a fault or discontinuity in the cable. The offset frequency required to maximize the second maximum response is proportional to the time delay (round trip) of the cable because the delayed signal appropriately offset in frequency relative to the local oscillator frequency is equivalent to the tracking generator's output of "zero offset" previously providing the first maximum response. Thus, plotting the display magnitude versus offset frequency provides information regarding the time delay associated with the cable's discontinuity.
The resolution of distance information, while correctly measuring the magnitude of the reflected signal, is limited by the use of a bandpass filter in the manual procedure. The measurement must be made in the length of time the spectrum analyzer takes to sweep. If a wide bandwidth is used, the spectrum analyzer responds quickly to the returning signal, reaching full amplitude before the sweep ends. However, the ability to determine exactly the frequency of the returning signal is limited because it could be anywhere in the central portion of the filter's bandpass. If a narrow bandpass filter is used in an attempt to reduce the frequency uncertainty, then the filter's risetime is too slow for the returning signal to reach full amplitude before the sweep ends. Additionally this conventional manual measurement includes some perceptual errors of an operator.
Therefore, what is desired is to provide an improved frequency domain reflectometry technique capable of automatically and precisely locating a fault or discontinuity point of a cable without being limited to the minimum resolution of the spectrum analyzer.